Score : $300$ points

问题描述

给定一个包含非负整数的长度为 $N$ 的序列 $A=(A_1,A_2,\ldots,A_N)$。

确定是否存在一个可以用 $A$ 中两个不同元素之和表示的偶数。如果存在，找出这样的最大数。

以上为通义千问 qwen-max 翻译，仅供参考。

Problem Statement

You are given a sequence $A=(A_1,A_2,\ldots,A_N)$ of length $N$ consisting of non-negative integers.

Determine if there is an even number represented as the sum of two different elements of $A$. If it exists, find the maximum such number.

Constraints

• $2\leq N \leq 2\times 10^5$
• $0\leq A_i\leq 10^9$
• The elements of $A$ are distinct.
• All values in the input are integers.

Input

The input is given from Standard Input in the following format:

$N$

$A_1$ $A_2$ $\ldots$ $A_N$

Output

Print -1 if there is no even number represented as the sum of two different elements of $A$.

If such an even number exists, print the maximum such number.

Sample Input 1

3
2 3 4

Sample Output 1

6

The values represented as the sum of two distinct elements of $A$ are $5$, $6$, and $7$. We have an even number here, and the maximum is $6$.

Sample Input 2

2
1 0

Sample Output 2

-1

The value represented as the sum of two distinct elements of $A$ is $1$. We have no even number here, so -1 should be printed.

update @ 2024/3/10 11:27:35